Ultrasonic actuator

ABSTRACT

An ultrasonic actuator ( 3 ) includes an actuator body ( 4 ) performing a plurality of vibrations including a bending vibration, and a driver element ( 5 ) which is attached to a long side surface ( 40   b ) of the actuator body ( 4 ), and outputs a driving force by making an orbit motion in response to the vibrations of the actuator body ( 4 ). The driver element ( 5 ) is provided with an attachment surface ( 51 ), and is attached to the long side surface ( 40   b ) with the attachment surface ( 51 ) in surface contact with the long side surface ( 40   b ). A width of the attachment surface ( 51 ) in the longitudinal direction of the long side surface ( 40   b ) is smaller than a maximum width of the driver element ( 5 ) in the longitudinal direction of the long side surface ( 40   b ).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT International PatentApplication No. PCT/JP2008/000598, filed on Mar. 14, 2008, which claimspriority of Japanese Patent Application No. 2007-066587, filed on Mar.15, 2007, the entire contents of which are expressly incorporated byreference herein.

TECHNICAL FIELD

The present invention relates to an ultrasonic actuator including anactuator body performing a plurality of vibrations including a bendingvibration.

BACKGROUND

Conventionally, an ultrasonic actuator disclosed in Patent Document 1has been known as an ultrasonic actuator including an actuator bodyperforming a plurality of vibrations including a bending vibration.

The ultrasonic actuator of Patent Literature 1 includes an actuator bodyconstituted of a piezoelectric element, and a driver element attached tothe actuator body.

The actuator body is constituted of a flat plate-shaped piezoelectricelement having a longitudinal direction. In the actuator body, alongitudinal vibration (i.e., a so-called expansion/contractionvibration) in the longitudinal direction of the piezoelectric elementand a bending vibration in a lateral direction of the piezoelectricelement are induced harmonically by respectively applying AC voltageshaving different phases to two electrode pairs, each of which includesdiagonally aligned two electrodes. As a result, the driver element makesan orbit motion, more specifically, an elliptical motion, in a planeincluding the longitudinal direction and the lateral direction of thepiezoelectric element.

The driver element is substantially in the shape of a hemisphere. Twodriver elements are arranged on a long side surface of the actuatorbody. The long side surface is a surface which is normal to a vibrationdirection of the bending vibration of the actuator body, and is bent anddeformed in response to the bending vibration of the actuator body. Thetwo driver elements are arranged at positions on the long side surfacecorresponding to antinodes of the bending vibration where bendingdisplacement is the maximum.

In the ultrasonic actuator thus configured, the driver elements arearranged in contact with the drive target. When the ultrasonic actuatoris driven in this state, a driving force of the actuator body istransmitted to the drive target through the driver elements making theorbit motion as described above, and therefore the drive target isdriven. At this time, the actuator body is biased toward the drivetarget, and is configured so that a friction force between the driverelements and the drive target is increased to efficiently transmit thedriving force of the actuator body.

Patent Document 1: Published Japanese Patent Application No. 2004-304963SUMMARY

However, similar to the ultrasonic actuator of Patent Document 1, in thestructure in which the driver elements are arranged on the side surfacenormal to the vibration direction of the bending vibration of theactuator body, i.e., the surface to be bent and deformed, the bendingand deformation of the side surface is affected by the driver elementsmore significantly as a contact area between the driver elements and theside surface becomes greater.

On the other hand, as the contact area between the driver elements andthe side surface becomes smaller, the attachment of the driver elementto the side surface becomes less firm. As described above, in thestructure in which the driving force of the actuator body is transmittedto the drive target by the friction force between the driver elementsand the drive target, firm attachment is required between the driverelements and the actuator body.

An object of the present invention is to simultaneously allow bothsuppressing the disturbance of the vibrations of the actuator body bythe driver elements, and firmly attaching the driver elements to theactuator body.

To achieve the object, an ultrasonic actuator includes: an actuator bodyhaving a piezoelectric element and performing a bending vibration and alongitudinal vibration; and a driver element which is attached to a sidesurface of the actuator body normal to a vibration direction of thebending vibration, and outputs a driving force by making an orbit motionin a plane including the vibration direction of the bending vibrationand a vibration direction of the longitudinal vibration in response tothe vibrations of the actuator body, wherein the driver element isprovided with an attachment surface, and is attached to the side surfacewith the attachment surface in surface contact with the side surface,and a width of the attachment surface in the vibration direction of thelongitudinal vibration is smaller than a maximum width of the driverelement in the vibration direction of the longitudinal vibration.

The width of the attachment surface in the vibration direction of thelongitudinal vibration is smaller than the maximum width of the driverelement in the vibration direction of the longitudinal vibration so asto reduce a contact area between the driver element and the actuatorbody. As a result, the disturbance of the vibrations of the actuatorbody by the driver element can be suppressed. Further, since the driverelement and the actuator body are brought into surface contact, not intopoint contact or line contact, to keep a sufficient contact area, firmattachment of the driver element can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating the structure of anultrasonic actuator according to Embodiment 1.

FIG. 2 is a sectional view illustrating the structure of the ultrasonicactuator.

FIG. 3 is a perspective view of a driver element.

FIG. 4 is a conceptual diagram illustrating displacement made by a firstmode of longitudinal vibration of a piezoelectric element.

FIG. 5 is a conceptual diagram illustrating displacement made by asecond mode of bending vibration of the piezoelectric element.

FIG. 6 is a conceptual diagram illustrating the operation of thepiezoelectric element.

FIG. 7 is a sectional view illustrating the dimensions of the driverelement and a disc body.

FIG. 8 is a graph illustrating displacement made by the bendingvibration and the rate of change in displacement.

FIG. 9 is an exploded perspective view illustrating the structure of anultrasonic actuator according to Embodiment 2.

FIG. 10 is a perspective view of a driver element.

EXPLANATION OF REFERENCE NUMERALS

-   c Point of contact-   n Normal-   X Rotation axis-   2 Disc body (drive target)-   4, 204 Actuator body-   40, 240 a, 240 b Piezoelectric element-   40 b, 242 b Long side surface (side surface normal to vibration    direction of bending vibration)-   5, 205 Driver element-   51, 251 Attachment surface

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

Embodiment 1

A drive unit 1 according to Embodiment 1 of the present inventionincludes, as shown in FIGS. 1 and 2, a disc body 2, an ultrasonicactuator 3 for driving the disc body 2, and a control unit (not shown)for performing drive control to the ultrasonic actuator 3.

The disc body 2 is a disc having a rotation axis X as a center and madeof alumina or other material. The disc body 2 is arranged rotatablyaround the rotation axis X. The disc body 2 constitutes a drive target.

The ultrasonic actuator 3 includes an actuator body 4 for generating avibration, driver elements 5 for transmitting a driving force of theactuator body 4 to the disc body 2, a case 6 containing the actuatorbody 4, support rubbers 71 placed between the actuator body 4 and thecase 6 to elastically support the actuator body 4, and a bias rubber 72for biasing the actuator body 4 to the disc body 2.

The actuator body 4 includes a piezoelectric element 40 and power supplyelectrodes 41 a to 41 d formed on the piezoelectric element 40.

The piezoelectric element 40 is made of a piezoelectric material such aslead zirconate titanate or quartz, and shaped into a flat plate which isrectangular when viewed in plan. More specifically, the piezoelectricelement 40 includes a pair of substantially rectangular principalsurfaces 40 a opposing to each other (only one of them is shown in FIG.1), a pair of long side surfaces 40 b and 40 c which are orthogonal tothe principle surfaces 40 a, extend in a longitudinal direction of theprinciple surfaces 40 a, and oppose to each other, and a pair of shortside surfaces 40 d and 40 e which are orthogonal to both of theprinciple surfaces 40 a and the long side surfaces 40 b and 40 c, extendin a lateral direction of the principle surfaces 40 a, and oppose toeach other.

On the principle surface 40 a of the piezoelectric element 40, fourpower supply electrodes 41 a to 41 d are formed. More specifically,suppose that the principle surface 40 a is divided in four quadrants,i.e., two areas in the longitudinal direction and two areas in thelateral direction, the power supply electrodes 41 a to 41 d are formedon the four quadrants, respectively. Among the power supply electrodes41 a to 41 d, two diagonally aligned power supply electrodes, i.e., thepower supply electrodes 41 a and 41 d, are electrically connectedthrough a wire 9 a, and the power supply electrodes 41 b and 41 c areelectrically connected through a wire 9 b. Wires 9 c and 9 d areconnected to the power supply electrodes 41 c and 41 d, respectively,and the wires 9 c and 9 d are guided to the outside from through holes(not shown) provided in the case 6. That is, power can be supplied tothe power supply electrodes 41 b and 41 c through the wire 9 c, and tothe power supply electrodes 41 a and 41 d through the wire 9 d.

On the other principle surface of the piezoelectric element 40, a groundelectrode (not shown) is formed substantially on the whole surface. Awire 9 e is connected to the ground electrode. The wire 9 e is alsoguided to the outside from a through hole (not shown) provided in thecase 6.

The wires 9 a to 9 e are electrically connected to the correspondingelectrodes by solders. The solders are positioned near the nodes of afirst mode of a longitudinal vibration and a second mode of a bendingvibration of the piezoelectric element 40 to be described later. Thatis, by connecting the wires 9 a and 9 e near the nodes of the vibrationsof the piezoelectric element 40 by the solders, the disturbance of thevibrations of the piezoelectric element 40 by the solders can besuppressed as much as possible.

On the long side surface 40 b of the piezoelectric element 40, thedriver elements 5 are arranged to be spaced from each other in thelongitudinal direction of the long side surface 40 b (a directioncorresponding to a vibration direction of the longitudinal vibration tobe described later). The driver elements 5 are provided on parts of thelong side surface 40 b located at a distance of about 30% of the fulllength of the long side surface 40 b inwardly from both edges thereof inthe longitudinal direction, and on a center part of the long sidesurface 40 b in the lateral direction (a thickness direction of thepiezoelectric element 40). Each of the locations of the driver elements5 corresponds to an antinode of a second mode of a bending vibration tobe described later, where the bending vibration is the maximum. With thedriver elements 5 arranged at these locations, the bending vibration ofthe piezoelectric element 40 can be utilized more effectively. Thedriver elements 5 can be arranged on other parts of the piezoelectricelement than the nodes of the second mode of the bending vibration,i.e., non-node parts. As the non-node parts make displacement inresponse to the bending vibration, the bending vibration of thepiezoelectric element 40 can be utilized.

Each of the driver elements 5 is basically in the shape of a spherewhich is partially truncated to form a flat section, as shown in FIG. 3.The section constitutes a round attachment surface 51. Each of thedriver elements 5 is adhered to the long side surface 40 b of thepiezoelectric element 40 through the attachment surface 51 using anadhesive 52. The driver elements 5 may be made of zirconia, alumina,silicon nitride, silicon carbide, tungsten carbide, or other material.

The adhesive 52 is preferably softer than the material of thepiezoelectric element 40 and the material of the driver elements 5. Morespecifically, the adhesive may be a synthetic resin, especially an epoxyresin or a silicone resin. With use of these materials, the driverelements 5 and the piezoelectric element 40 can be fixed to each otherwithout affecting the bending vibration of the piezoelectric element 40as much as possible.

The case 6 is formed of resin, and is substantially U-shaped when viewedfrom the front so as to cover the other long side surface 40 c and theshort side surfaces 40 d and 40 e of the piezoelectric element 40. Thecase 6 includes a long side wall portion 61 which is substantiallyrectangular and parallel to the long side surface 40 c of thepiezoelectric element 40, and short side wall portions 62 provided atshort sides of the long side wall portion 61 at the longitudinal endsthereof, respectively. A recess 61 a in which a bias rubber 72 to bedescribed later will be arranged is formed in the longitudinal andlateral center of an inner surface of the long side wall portion 61.Further, a recess 62 a in which a support rubber 71 to be describedlater will be arranged is formed in the longitudinal and lateral centerof an inner surface of each of the short side wall portions 62.

The actuator body 4 is placed in the case 6 thus configured. Theactuator body 4 is placed in the case 6 so that the other long sidesurface 40 c of the piezoelectric element 40 opposes to the long sidewall portion 61. In this state, the driver elements 5 protrude from thecase 6.

The support rubbers 71 are provided between the short side surfaces 40 dand 40 e of the piezoelectric element 40 and the short side wallportions 62 of the case 6, respectively. Each of the support rubbers 71is fitted in the recess 62 a in the short side wall portion 62 and is incontact with the short side surface 40 d (40 e) of the piezoelectricelement 40. The short side surfaces 40 d and 40 e of the piezoelectricelement 40 correspond to nodes of the longitudinal vibration to bedescribed later. However, since the support rubbers 71 are elasticbodies, they can support the piezoelectric element 40 without affectingthe longitudinal vibration of the piezoelectric element 40.

A bias rubber 72 is placed between the other long side surface 40 c andthe long side wall portion 61 of the case 6. The bias rubber 72 isfitted in the recess 61 a in the long side wall portion 61 and is incontact with the long side surface 40 c of the piezoelectric element 40.

The support rubbers 71 and the bias rubber 72 are made of siliconerubber or other material.

In the ultrasonic actuator 3 this configured, the wires 9 c, 9 d and 9 eare connected to the control unit. With the wire 9 e connected to theelectrical ground, an AC voltage of a predetermined frequency is appliedto the wire 9 c, and an AC voltage having a phase shifted from the ACvoltage applied to the wire 9 by 90° is applied to the wire 9 d. As aresult, AC voltages shifted from each other by 90° are applied to a pairof diagonally aligned power supply electrodes 41 a and 41 d and anotherpair of diagonally aligned power supply electrodes 41 b and 41 c on theprinciple surface 40 a of the piezoelectric element 40, respectively.This induces the piezoelectric element 40 to generate the first mode oflongitudinal vibration (so-called expansion/contraction vibration) shownin FIG. 4 and the second mode of bending vibration shown in FIG. 5. Thatis, the piezoelectric element 40 performs the longitudinal vibration inthe longitudinal direction of the long side surface 40 b and the bendingvibration in the normal direction of the long side surface 40 b.

Respective resonance frequencies of longitudinal vibration and bendingvibration are determined by a material, a shape and the like of thepiezoelectric element 40. Furthermore, the resonance frequencies bothare influenced by force supporting the piezoelectric element 40 and aposition where the piezoelectric element 40 is supported. Taking thisinto consideration, the resonance frequencies are substantially matchedto each other. AC voltages having a frequency around the resonancefrequencies and phases shifted by 90° from each other are applied to thewires 9 c and 9 d, respectively. Thus, the first mode of longitudinalvibration and the second mode of bending vibration are harmonicallyinduced in the piezoelectric element 40, so that the piezoelectricelement 40 changes itself into shapes shown in FIGS. 6( a), 6(b), 6(c)and 6(d) in this order.

As a result, each of the driver elements 5 provided on the piezoelectricelement 40 makes an orbit motion, more specifically, a substantiallyelliptical motion, in a plane parallel to a principal surface 40 a ofthe piezoelectric element 40 (i.e., a plane parallel to the page surfaceof FIG. 6), i.e., in a plane including the longitudinal direction (avibration direction of the longitudinal vibration) and the normaldirection (a vibration direction of the bending vibration) of the longside surface 40 b of the piezoelectric element 40.

The ultrasonic actuator 3 thus configured is arranged such that thedriver elements 5 are in contact with a circumferential surface of thedisc body 2. That is, the ultrasonic actuator 3 is arranged so that theplane including the longitudinal and normal directions of the long sidesurface 40 b of the piezoelectric element 40, i.e., the plane includingthe vibration directions of the longitudinal and bending vibrations, isorthogonal to the rotation axis X of the disc body 2. In this state,sections of the disc body 2 and the driver elements 5 which areorthogonal to the rotation axis X of the disc body 2, i.e., sectionsparallel to the plane including the longitudinal direction of the longside surface 40 b of the piezoelectric element 40 (the vibrationdirection of the longitudinal vibration) and the vibration direction ofthe bending vibration, are substantially round (in a strict sense, thesection of each of the driver elements 5 is almost round-shaped with acertain circular segment part cut off).

In this structure, the ultrasonic actuator 3 is in such a state that thebias rubber 72 is compressed and deformed to bias the driver elements 5to the disc body 2 by the elastic force of the bias rubber 72. That is,the driver elements 5 are biased to the disc body 2 even when theultrasonic actuator 3 is not driven. Since the bias rubber 72 isarranged on a perpendicular bisector of a line segment connecting thecenters of the driver elements 5, and the driver elements 5 are biasedin the direction of the perpendicular bisector by the bias rubber 72,the two driver elements 5 are biased to the disc body 2 withsubstantially the same biasing forces. Thus, the disc body 2 can bedriven by the two driver elements 5 in good balance with stability.

In this state, as described above, the wire 9 c is connected to theelectrical ground, an AC voltage at a predetermined frequency is appliedto the wire 9 d, and an AC voltage having a phase shifted from the ACvoltage applied to the wire 9 d by 90° C. is applied to the wire 9 e. Asa result, the ultrasonic actuator 3 induces the piezoelectric element 40to generate a composite vibration of the longitudinal vibration and thebending vibration, so that the driver elements 5 make a substantiallyelliptical motion in a plane parallel to the principle surface 40 a ofthe piezoelectric element 40. In this way, the driver elements 5periodically repeat contacting to and detaching from the disc body 2, sothat the disc body 2 is rotated about the rotation axis X by frictionforce. That is, the ultrasonic actuator 3 applies a driving force to thedisc body 2 in the circumferential direction thereof. The two driverelements 5 make the substantially elliptical motion with their phasesshifted by 180° from each other as shown in FIG. 6. Therefore, thedriver elements 5 alternately drive the disc body 2.

For the purpose of improving firm attachment to the piezoelectricelement 40, the driver elements 5 are adhered to the long side surface40 b of the piezoelectric element 40 with the attachment surface 51 insurface contact with long side surface 40 b. However, as the contactarea between the driver element 5 and the long side surface 40 b becomesgreater, the driver elements 5 restrain the displacement of the longside surface 40 b in a wider range, and the vibration of thepiezoelectric element 40 is affected more significantly. Therefore, asshown in FIG. 7, suppose that the radius of the driver element 5 (theradius of the substantially spherical body) is r, the width of theattachment surface 51 in the longitudinal direction of the long sidesurface 40 b (the vibration direction of the longitudinal vibration),i.e., the diameter x of the attachment surface 51 satisfies thefollowing expression (1):

x<2r  (1).

Specifically, the width of the attachment surface 51 in the vibrationdirection of the longitudinal vibration (the diameter of the attachmentsurface 51) is defined smaller than the maximum width of the driverelement 5 in the vibration direction of the longitudinal vibration (thediameter of the driver element 5).

However, in the structure similar to that described in the presentembodiment in which a normal n at a point of contact c between thedriver element 5 and the disc body 2 is inclined relative to the normaldirection of the long side surface 40 b in the vibration direction ofthe longitudinal vibration, a reaction force from the disc body 2 isapplied to the driver element 5 in the direction of the normal n. In thecase where the normal n passes through the attachment surface 51 of thedriver element 5, the reaction force from the disc body 2 is exerted onthe attachment surface 51 of the driver element. Part of the reactionforce is exerted in the shearing direction of the attachment surface 51and the long side surface 40 b of the piezoelectric element 40, and therest of which is exerted in the perpendicular direction of theattachment surface 51 and the long side surface 40 b, i.e., thedirection pressing the driver element 5 against the long side surface 40b. Thus, the reaction force can be received by the attachment surface 51and the long side surface 40 b. In contrast, in the case where thenormal n does not pass through the attachment surface 51 of the driverelement 5 but passes through the spherical surface of the driver element5, the reaction force from the disc body 2 is exerted so as to raise theattachment surface 51 above the long side surface 40 b with a point onthe circumference of the attachment surface 51 as a fulcrum. Therefore,the driver element 5 may possibly come off the long side surface 40 b.

Although the driver element 5 is in surface contact with and adhered tothe long side surface 40 b, it is preferable, for reliably preventingthe driver element 5 from coming off the long side surface 40 b, todefine the diameter of the driver element 5 and the size of theattachment surface 51 so that the normal n at the point of contact cbetween the driver element 5 and the disc body 2 passes through theattachment surface 51 of the driver element 5.

In other words, suppose that the radius of the disc body 2 is R, and thedistance between the centers of the two driver elements 5 in thelongitudinal direction of the long side surface 40 b (the vibrationdirection of the longitudinal vibration) is l, it is preferable that thediameter x of the attachment surface 51 satisfies the followingexpression (2):

x>rl/(R+r)  (2).

By meeting the expression, the reaction force from the disc body 2 canbe received by the long side surface 40 b of the piezoelectric element40 through the attachment surface 51 of the driver element 5. Thisallows preventing the driver element 5 from coming off the long sidesurface 40 b.

However, even in the case where the normal n passes through theattachment surface 51 of the driver element 5, when the normal n at theattachment surface 51 of the driver element 5 is inclined too muchrelative to the normal of the long side surface 40 b, part of thereaction force from the disc body 2 exerted in the shearing direction ofthe attachment surface 51 and the long side surface 40 b of thepiezoelectric element 40 may become greater than the other part of thereaction force from the disc body 2 exerted in the perpendiculardirection of the attachment surface 51 and the long side surface 40 b.For this reason, it is preferable that the diameter x of the attachmentsurface 51 satisfies the following expression (3):

x<√{square root over (2)}r  (3).

By meeting the expression, the part of the reaction force from the discbody 2 exerted in the shearing direction of the attachment surface 51and the long side surface 40 b is reduced so that the driver element 5can reliably be prevented from coming off the long side surface 40 b.

The displacement made by the bending vibration of the piezoelectricelement 40 is as shown in FIG. 8. In FIG. 8, a lateral axis indicatesthe position of the long side surface 40 b in the longitudinaldirection, which is a relative position with respect to the full lengthof the long side surface 40 b regarded as 1. A longitudinal axisindicates the displacement made by the bending vibration. A solid linein this figure denotes the displacement made by the bending vibration,in which extreme values are observed when the relative position isaround 0.3 and 0.7. In order to efficiently transmit the bendingvibration of the piezoelectric element 40 to the disc body 2, the driverelements 5 are preferably arranged at positions where the bendingdisplacement is large, i.e., positions corresponding to the extremevalues. Further, when adhered to the long side surface 40 b, the driverelements 5 restrain the deformation of the long side surface 40 b.Therefore, the driver elements 5 are preferably arranged at positionswhere the rate of change in bending displacement is low. A broken linein this figure denotes the rate of change in bending displacement, inwhich the rate of change in bending displacement is low around theextreme values, but increased as it is shifted from the extreme values.Therefore, even when the driver elements 5 are arranged at positionscorresponding to the extreme values, the size of the attachment surface51 is preferably defined within a range where the rate of change inbending displacement is low. Specifically, suppose that the full lengthof the long side surface 40 b in the longitudinal direction is L, it ispreferable that the diameter x of the attachment surface 51 satisfiesthe following expression (4):

x<L/10  (4).

By meeting the expression, the driver element 5 can be attached to partof the long side surface 40 b where the rate of change in bendingvibration is low. As a result, the disturbance of the bending vibrationof the piezoelectric element 40 by the adhesion of the driver element 5to the long side surface 40 b can further be prevented.

As the normal n at the point of contact between the driver element andthe disc body 2 is more inclined relative to the normal direction of thelong side surface 40 b, i.e., to the vibration direction of the bendingvibration, a period during which the driving force can be applied to thedisc body 2 in the elliptical motion of the driver element 5 becomesshorter. Therefore, it is not preferable that the normal n is inclinedvery much relative to the normal of the long side surface 40 b. Thus, itis preferable that the radius r of the driver element 5 satisfies thefollowing expression (5):

$\begin{matrix}{r > {\frac{l}{\sqrt{2}} - {R.}}} & (5)\end{matrix}$

By meeting the expression, the driving force generated by the actuatorbody 4 can efficiently be transmitted to the disc body 2.

According to Embodiment 1, in the structure in which the driver elements5 are attached to the long side surface 40 b of the piezoelectricelement 40, which is the side surface which makes the bendingdeformation, the width of the attachment surface 51 of each of thedriver elements 5 in the vibration direction of the longitudinalvibration, i.e., the diameter x, is defined smaller than the width ofeach of the driver elements 5 in the vibration direction of thelongitudinal vibration, i.e., the diameter 2r, so that the contact areabetween each of the driver elements 5 and the long side surface 40 b isreduced. This makes it possible to suppress the disturbance of thebending deformation of the long side surface 40 b by the driver elements5. Specifically, since the vibration direction of the longitudinalvibration (the longitudinal direction of the long side surface 40 b) isa direction in which the bending vibration spreads (propagates), therestraint of the long side surface 40 b by the attachment surface 51 isreduced by reducing the width of the attachment surface 51 in thevibration direction of the longitudinal vibration.

Although the contact area between the driver element 5 and the long sidesurface 40 b is reduced, they are not brought into point contact or linecontact. Instead, the driver element 5 is provided with the attachmentsurface 51 so that the driver element 5 is attached to the long sidesurface 40 b with the attachment surface 51 in surface contact with thelong side surface 40 b. This allows securing firm attachment of thedriver element 5 to the long side surface 40 b. That is, both reduceddisturbance of the vibration of the actuator body 4 by the driverelement 5, and firm attachment of the driver element 5, can be achievedsimultaneously.

In the structure in which the driver elements 5 are brought into contactwith a surface not parallel to the long side surface 40 b on which thedriver elements 5 are arranged, e.g., the circumferential surface of thedisc body 2, a reaction force exerted on the driver elements 5 by thedrive target (the disc body 2) is inclined relative to the normaldirection of the long side surface 40 b. Therefore, the degree ofattachment of the driver elements 5 to the long side surface 40 b is agreat concern. Specifically, in the structure in which the reactionforce from the drive target is exerted on the driver elements 5 and thelong side surface 40 b in the direction inclined relative to the normaldirection of the long side surface 40 b, the structure in which thedriver elements 5 are attached to the long side surface 40 b with theattachment surfaces 51 in surface contact with the long side surface 40b is particularly effective, as described above.

The size of the attachment surface 51 of the driver element 5, morespecifically, the width thereof in the vibration direction of thelongitudinal vibration, is defined so that the normal n at the point ofcontact c between the driver element 5 and the disc body 2 passesthrough the attachment surface 51. As a result, the reaction forceexerted on the driver element 5 by the disc body 2 can be received bythe attachment surface 51 and the long side surface 40 b, and the driverelement 5 can be prevented from coming off the long side surface 40 b.

Further, when the reaction force is exerted on the driver element 5 bythe disc body 2, part of the reaction force exerted in the shearingdirection of the attachment surface 51 and the long side surface 40 b isprevented from becoming greater than the other part of the reactionforce exerted in the perpendicular direction of the attachment surface51 and the long side surface 40 b by defining the radius r of the driverelement 5 and the diameter x of the attachment surface 51 so as tosatisfy the expression (3). This makes it possible to reliably preventthe driver element 5 from coming off the long side surface 40 b.

Further, by arranging the driver elements 5 at positions on the longside surface 40 b corresponding to the antinodes of the bendingvibration of the piezoelectric element 40, the driver elements 5 can beattached to the positions on the long side surface 40 b where the rateof change in bending displacement is small. Therefore, the disturbanceof the bending deformation of the long side surface 40 b by the driverelements 5 can be suppressed to a further extent.

Embodiment 2

A drive unit according to Embodiment 2 of the present invention will bedescribed. In a drive unit 201 according to Embodiment 2, the structureof an ultrasonic actuator 203, particularly the structures of anactuator body 204 and a driver element 205, is different from thestructure of the ultrasonic actuator 3 of Embodiment 1.

Specifically, as shown in FIG. 9, the actuator body 204 includes aresonator 242, piezoelectric elements 240 a and 240 b arranged in theresonator 242, and power supply electrodes (not shown) attached to thepiezoelectric elements 240 a and 240 b, respectively.

The resonator 242 is a substantially in the shape of a rectangularparallelepiped made of metal such as stainless steel and an aluminumalloy, or an insulating material such as ceramic. The resonator 242 hastwo arrangement holes 242 c and 242 d aligned in the longitudinaldirection at one lateral end of a substantially rectangular principlesurface 242 a (one of the two surfaces having the largest area among thesix surfaces).

Each of the piezoelectric elements 240 a and 240 b is made of apiezoelectric material such as lead zirconate titanate or quartz, andshaped into a flat plate which is rectangular when viewed in plan. Thepiezoelectric element 240 a (240 b) has power supply electrodes (notshown) formed uniformly on the opposing surfaces thereof. Thepiezoelectric elements 240 a and 240 b are contained in the arrangementholes 242 c and 242 d of the resonator 242, respectively.

Each of the driver elements 205 is basically in the shape of acylindrical column which is partially truncated to form a flat sectionparallel to the axis of the cylindrical column, as shown in FIG. 10. Theflat section constitutes a rectangular attachment surface 251. Each ofthe driver elements 205 is adhered to a long side surface 242 b (a sidesurface which is orthogonal to the principle surface 242 a and extendsin the longitudinal direction of the principle surface 242 a) of theresonator 242 through the attachment surface 251 using an adhesive 52.The driver elements 205 may be made of zirconia, alumina, siliconnitride, silicon carbide, tungsten carbide, or other material.

The driver elements 205 are provided on parts of the long side surface242 b located at a distance of about 30% of the full length of the longside surface 242 b inwardly from both edges thereof in the longitudinaldirection, with the axis of the cylindrical column parallel to a lateraldirection of the long side surface 242 b (a thickness direction of theresonator 242). Each of the locations of the driver elements 205corresponds to an antinode of a second mode of a bending vibration to bedescribed later, where the bending vibration is the maximum.

By applying predetermined AC voltages to the piezoelectric elements 240a and 240 b, respectively, the resonator 242 is induced to harmonicallygenerate a longitudinal vibration and a bending vibration. Morespecifically, by alternately allowing the piezoelectric elements 240 aand 240 b to generate the longitudinal vibration at the predeterminedfrequencies, the resonator 242 performs the longitudinal vibration inthe longitudinal direction of the piezoelectric elements 240 a and 240b, i.e., the longitudinal direction of the resonator 242. At the sametime, since the piezoelectric elements 240 a and 240 b are arranged onlyon the one lateral end of the resonator 242, one side in thelongitudinal direction (a side corresponding to the piezoelectricelement 240 a) and the other side in the longitudinal direction (a sidecorresponding to the piezoelectric element 240 b) of the one lateral endof the resonator 242 expand and contract alternately. As a result, theresonator 242 performs the bending vibration in the lateral direction.

Respective resonance frequencies of the longitudinal vibration and thebending vibration are determined by a material, a shape and the like ofthe resonator 242. Furthermore, the resonance frequencies are bothinfluenced by a force supporting the resonator 242 and a position wherethe resonator 242 is supported. Taking this into consideration, theresonance frequencies are made to substantially match each other. ACvoltages having a frequency around the resonance frequencies and phasesshifted by 90° from each other are applied to the piezoelectric elements240 a and 240 b, respectively. Thus, the first mode of longitudinalvibration and the second mode of bending vibration are harmonicallyinduced in the resonator 242, so that the resonator 242 changes itselfinto shapes shown in FIGS. 6( a), 6(b), 6(c) and 6(d) in this order, inthe same manner as the actuator body 4 of Embodiment 1. As a result,each of the driver elements 205 provided on the resonator 242 makes anorbit motion, more specifically, a substantially elliptical motion, in aplane including the longitudinal direction of the long side surface 242b of the resonator 242 (the vibration direction of the longitudinalvibration) and the normal direction of the long side surface 242 b (thevibration direction of the bending vibration).

Just like the sections of the driver elements 5 of Embodiment 1,sections of the driver elements 205 which are orthogonal to the rotationaxis X of the disc body 2, i.e., sections parallel to the planeincluding the longitudinal direction of the long side surface 242 b ofthe resonator 242 (i.e., the vibration direction of the longitudinalvibration) and the vibration direction of the bending vibration, aresubstantially round (in a strict sense, the sections of each of thedriver element 205 is almost round-shaped with a certain circularsegment part cut off). The driver elements 205 are also configured tosatisfy the above-described expressions (1) to (5). Regarding theexpressions (1) to (5), r denotes the radius of the driver element 205basically in the shape of a cylindrical column, x denotes the width ofthe attachment surface 251 of the driver element 205 in the directionorthogonal to the axis of the cylindrical column, i.e., the width in thevibration direction of the longitudinal vibration of the resonator 242.

According to Embodiment 2, just as described in Embodiment 1, in thestructure in which the driver elements 205 are attached to the long sidesurface 242 b of the resonator 242, which is the side surface whichmakes the bending deformation, the width x of the attachment surface 251of each of the driver elements 205 in the vibration direction of thelongitudinal vibration is defined smaller than the width of each of thedriver elements 205 in the vibration direction of the longitudinalvibration, i.e., the diameter 2r, so that the contact area between eachof the driver elements 205 and the long side surface 242 b is reduced.This makes it possible to suppress the disturbance of the bendingdeformation of the long side surface 242 by the driver elements 205.Although the contact area between the driver element 205 and the longside surface 242 b is reduced, they are not brought into point contactor linear contact. Instead, the driver element 205 is provided with theattachment surface 251 so that the driver element 205 is attached to thelong side surface 242 b with the attachment surface 251 in surfacecontact with the long side surface 242 b. This allows securing firmattachment of the driver element 205 to the long side surface 242 b.

Except for the feature described above, the advantages and effects ofEmbodiment 2 are similar to those of Embodiment 1.

With use of the cylindrical columnar driver elements 205, the contactarea with the disc body 2 is increased. Therefore, the shape of thedriver elements 205 is less likely to be changed due to wear, and theultrasonic actuator can be provided with high reliability.

Other Embodiments

The above-described embodiments of the present invention may beconfigured as follows.

Embodiment 1 has described the ultrasonic actuator 3 in which the driverelements 5, each of which is substantially in the shape of a spherewhich is at least partially truncated to form a flat section, areattached to the piezoelectric element 40. Further, Embodiment 2 hasdescribed the ultrasonic actuator 203 in which the driver elements 205,each of which is substantially in the shape of a cylindrical columnwhich is at least partially truncated to form a flat section parallel tothe axis of the cylindrical column, are attached to the resonator 242including the piezoelectric elements 240 a and 240 b. However, thepresent invention is not limited thereto. For example, an ultrasonicactuator including the substantially cylindrical columnar driverelements 205 attached to the piezoelectric element 40, and an ultrasonicactuator including the substantially spherical driver elements 5attached to the resonator 242 may be possible. The substantiallyspherical driver element 5 which is at least partially truncated to forma flat section may additionally be cut at other part, e.g., at partthereof in contact with the drive target. Likewise, the substantiallycylindrical columnar driver element 205 which is at least partiallytruncated to form a flat section parallel to the axis of the cylindricalcolumn may additionally be cut at other part, e.g., at part thereof incontact with the drive target.

According to Embodiments 1 and 2, the piezoelectric element 40 or theresonator 242 constituting the actuator body is in the shape of arectangular parallelepiped which is rectangular when viewed from thefront. However, their shapes are not limited thereto. It is not alwaysnecessary that the piezoelectric element or the resonator is rectangularwhen viewed in plan, or in the shape of a rectangular parallelepiped.The present invention can be applied to any ultrasonic actuator, as longas the driver element is attached to a side surface of the actuator bodyfor performing the bending and longitudinal vibrations which is normalto the vibration direction of the bending vibration.

Further, according to Embodiments 1 and 2, the driver elements 5 (205)are brought into contact with the circumferential surface of the discbody 2. However, the drive target is not limited to the disc body. Forexample, the driver target may be in the shape of a flat plate, and thedriver elements 5 (205) may be brought into contact with a flat surfaceof the flat-plate drive target. In the structure in which the driverelements 5 (205) are brought into contact with a surface parallel to thelong side surface 40 b (242 b) on which the driver elements 5 (205) arearranged, such as the flat surface of the flat-plate drive target, thebiasing force of the bias rubber 72 is exerted on a point of contactbetween each driver element 5 (205) and the flat surface of the drivetarget in a direction perpendicular to the long side surface 40 b (242b). In driving, a friction force is also applied to the point of contactbetween the driver element 5 (205) and the flat surface of theflat-plate drive target in the driving direction. The resultant force ofthem is exerted on a surface of connection between the driver element 5(205) and the long side surface 40 b (242 b) in a direction notperpendicular to the long side surface 40 b (242 b). Therefore, thepresent invention is also effective for the structure in which thedriver elements 5 (205) are brought into contact with the flat surfaceof the flat-plate drive target.

According to Embodiments 1 and 2, the circumferential surface of thedisc body 2 with which the driver elements 5 (205) are brought intocontact is parallel to the long side surface 40 b (242 b) in a directionorthogonal to the vibration direction of the longitudinal vibration andthe vibration direction of the bending vibration. Therefore, thereaction force from the disc body 2 is exerted orthogonally to the longside surface 40 b (242 b) in the direction orthogonal to the vibrationdirection of the longitudinal vibration and the vibration direction ofthe bending vibration. However, the present invention is not limitedthereto. Specifically, just like in the case where the drive target isspherical, and the ultrasonic actuator 3 (203) is arranged relative tothe drive target so that a normal at a point of contact between thedriver element 5 (205) and the drive target is inclined relative to thelong side surface 40 b (242 b) in the direction orthogonal to thevibration direction of the longitudinal vibration and the vibrationdirection of the bending vibration, the reaction force from the drivetarget may be inclined relative to the long side surface 40 b (242 b)not only in the vibration direction of the longitudinal vibration, butalso in the direction perpendicular to the vibration direction of thelongitudinal vibration and the vibration direction of the bendingvibration. Even in this case, the attachment surfaces 51 (251) of thedriver elements 5 (205) have a width not only in the vibration directionof the longitudinal vibration, but also in the direction perpendicularto the vibration direction of the longitudinal vibration and thevibration direction of the bending vibration. Therefore, firm attachmentof the driver elements 5 (205) to the long side surface 40 b (242 b) canbe achieved also in the direction perpendicular to the vibrationdirection of the longitudinal vibration and the vibration direction ofthe bending vibration, and the driver elements 5 (205) can be preventedfrom coming off the long side surface 40 b (242 b).

According to Embodiments 1 and 2, the sections of the driver elementswhich are parallel to the plane including the vibration direction of thelongitudinal vibration and the vibration direction of the bendingvibration are substantially round. However, the present invention is notlimited thereto. Specifically, the section of the driver element is notnecessarily round. The section of the driver element may have any othershape, e.g., it may be in the shape of a polygon, or a combination of apolygon and a circular segment. Even in this case, the width of theattachment surface of the driver element in the vibration direction ofthe longitudinal vibration is defined smaller than the maximum width ofthe driver element in the vibration direction of the longitudinalvibration.

According to Embodiments 1 and 2, power is supplied to the piezoelectricelements through the wires. However, the present invention is notlimited thereto, and the power supply can be performed in any othermethod. For example, the support rubbers and the bias rubber are made ofconductive rubber, so that the power supply to the piezoelectric elementof the actuator body can be performed from the power supply electrodesformed on the case through the support rubbers and the bias rubber.

According to Embodiments 1 and 2, the first mode of the longitudinalvibration and the second mode of the bending vibration have beendescribed. However, the present invention can be applied to acombination of the first mode of the longitudinal vibration and thefourth mode of the bending vibration, a combination of other vibrations,or a combination of other modes.

The support members placed between the actuator body and the case areall made of elastic bodies. However, at least one of them may be theelastic body. The similar effects can also be obtained when a singleannular support member is arranged to surround the piezoelectricelement, and part of the support member to be connected to thepiezoelectric element is made of the elastic body.

According to the embodiments described above, a single-plate structurein which power supply elements are formed on the surfaces of thepiezoelectric element has been adopted. However, the similar advantagesand effects can be obtained by a layered structure in which electrodelayers and piezoelectric layers are stacked. In this case, an externalelectrode in electrical conduction with the electrode layers is formedon an outer surface of the layered structure. Then, a wire is connectedto the external electrode, or a support rubber and a bias rubber made ofconductive rubber are brought into contact with the external electrode,so as to apply a voltage to the electrode layers.

The structure of the power supply electrodes of the piezoelectricelement 40 described in Embodiment 1 is the simplest one includingfour-piece electrodes formed on four divided areas of one of theprinciple surfaces and a full surface electrode formed on the otherprinciple surface. However, the similar advantages and effects can beobtained even when the power supply electrodes are constituted oftwo-piece electrodes or five-piece electrodes formed on two or fivedivided areas of the principle surface.

The above embodiments are merely preferred embodiments in nature, andare not intended to limit the scope, applications and use of theinvention.

As described above, the present invention is useful for an ultrasonicactuator including an actuator body performing a plurality of vibrationsincluding a bending vibration.

1. An ultrasonic actuator comprising: an actuator body having apiezoelectric element and performing a bending vibration and alongitudinal vibration; and a driver element which is attached to a sidesurface of the actuator body normal to a vibration direction of thebending vibration, and outputs a driving force by making an orbit motionin a plane including the vibration direction of the bending vibrationand a vibration direction of the longitudinal vibration in response tothe vibrations of the actuator body, wherein the driver element isprovided with an attachment surface, and is attached to the side surfacewith the attachment surface in surface contact with the side surface,and a width of the attachment surface in the vibration direction of thelongitudinal vibration is smaller than a maximum width of the driverelement in the vibration direction of the longitudinal vibration.
 2. Theultrasonic actuator of claim 1, wherein the driver element is in theshape of a sphere which is at least partially truncated to form a flatsection, and the section constitutes the attachment surface.
 3. Theultrasonic actuator of claim 1, wherein the driver element is in theshape of a cylindrical column which is at least partially truncated toform a flat section parallel to an axis of the cylindrical column, andthe section constitutes the attachment surface.
 4. The ultrasonicactuator of claim 1, wherein the driver element is attached to anon-node part of the bending vibration of the actuator body.
 5. Theultrasonic actuator of claim 4, wherein the driver element is attachedto an antinode of the bending vibration of the actuator body.
 6. Theultrasonic actuator of claim 1, wherein the driver element is in contactwith a circumferential surface of a drive target which rotates about arotation axis orthogonal to the plane including the vibration directionof the bending vibration and the vibration direction of the longitudinalvibration, and applies a driving force to the drive target.
 7. Theultrasonic actuator of claim 6, wherein the driver element is configuredso that a normal at a point of contact between the driver element andthe drive target passes through the attachment surface.
 8. Theultrasonic actuator of claim 6, wherein two driver elements are attachedto the side surface of the actuator body to be aligned in the vibrationdirection of the longitudinal vibration, sections of the driver elementsand the drive target which are parallel to the plane including thevibration direction of the bending vibration and the vibration directionof the longitudinal vibration are substantially round, and suppose thata distance between centers of the two driver elements in the vibrationdirection of the longitudinal vibration is l, a radius of the driverelement is r, a width of the attachment surface of the driver element inthe vibration direction of the longitudinal vibration is x, and a radiusof the drive target is R, the following expression (2):x>rl/(R+r)  (2) is satisfied.
 9. The ultrasonic actuator of claim 8,wherein the driver element satisfies the following expression (3):x<√{square root over (2)}r  (3).
 10. The ultrasonic actuator of claim 1,wherein the actuator body performs a first mode of the longitudinalvibration and a second mode of the bending vibration.